Axial beam time of flight mass spectrometer

ABSTRACT

A time of flight mass spectrometer wherein the ionizing electron beam is in axial alignment with the path of the ions produced thereby and wherein an electron multiplier is utilized to generate a high intensity electron beam for ionization. An open mesh control grid is utilized to control the axial dimension of the ionization region so that it can be maintained very small in order that the distance traveled by each of the ions to an ion detector at the opposite end of a drift tube is very nearly the same. This will result in a more accurate measurement of the mass of the ions since the difference in travel time to reception by the ion detector will be primarily because of difference in mass alone and not because of a difference in the distance traveled.

United States Patent SPBCTROMETER ionizing electron beam is in axial alignment with the pa th of the ions produced thereby and wherein an electron multiplier 4 Claims, 1 Drawing Fig.

is utilized to generate a high intensity electron beam for US. Cl 250/4L9 SB, ionization An open mesh Conn-0| grid is utilized to control the 250F111 TF axial dimension of the ionization region so that it can be main- Int. Cl H01 1 39/34 mi very small in order that the distance traveled by each f Field of Search 250/41 .9 the ions to an ion detector at the opposite end f d ift tube is SB, very nearly the same. This will result in a more accurate mea- R cud surement of the mass of the ions since the difference in travel 8 arenas I time to reception by the ion detector will be primarily because UNITED STATES PATENTS of difierence in mass alone and not because of a difference in 2,457,530 12/1948 Coggeshall et a1 250/4L9 the distance traveled.

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7 ..l m /z IMDVI' 1 J0 Ezra-(me I 4 ,a/i/J 501w Y ZfCW/Y I I mama/2 J1 T 420 i L F54 C'VZEE/VT' M/v/roe 1500 +4 Kv 42W 4 H V Q44! Inventor Marvin L. Vestal Baltimore, Md.

Appl. No 775,680

Filed Nov. 14, 1968 Patented June 22,1971.

Assignee Scientific Research Instruments Corporation Baltimore, Md.

AXIAL BEAM TIME OF FLIGHT MASS Primary Examiner-William F. Lindquist Attorney-Cushman, Darby and Cushman ABSTRACT: A time of flight mass spectrometer wherein the AXIAL BEAM TIME OF FLIGHT MASS SPECTROMETER BACKGROUND OF THE INVENTION The present invention relates to a time of flight mass spectrometer and more particularly to an improved instrument wherein an electron multiplier is utilized to produce the electron ionizing beam and wherein a control grid is utilized to accurately control the axial dimension of the ionization region.

In time of flight mass spectrometry various means have been devised whereby ions of like mass-to-charge ratio are separated into distinct spaced groups which impinge upon a collector electrode of the spectrometer at various times, depending upon the mass-to-charge ratio of the ions in each of the respective groups. For example, one way in which such groups of ions have been separated prior to striking the collector or ion detector of a spectrometer has been to increase the flight path of the ions from the ionization region to the ionization detector or the collector electrode. As a result, the light ions will reach the detector first, followed by the heavier ions. This has been accomplished in various ways as by increasing the length of the drift tube or by the utilization of a coil surrounding the drift tube so that a helical path, many times longer than the distance of the drift tube, is followed by the ions in their travel to the collector electrode. Although such devices have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reason that they require the use of more space, and the additional power requirement ofa coil surrounding the drift tube reduces the efficiency of the spectrometer.

SUMMARY OF THE INVENTION Accordingly, the general purpose of this invention is to provide a time of flight mass spectrometer which embraces all of the advantages of similarly employed spectrometers and possesses none of the aforedescribed disadvantages. To attain this the present invention contemplates the use of an electron multiplier to generate a high intensity ionizing electron beam and wherein a control grid is utilized to accurately regulate the bounds of the ionization region. In addition, the spectrometer of this invention operates without the need for accelerating grids located between the ionization region and the ion detector or collector electro'cle so that the undesirable background noise created by the presence of such grids is eliminated.

Accordingly, an object of the present invention is the provision of a time of flight mass spectrometer which provides for high ionization efficiency wherein a high intensity ionizing electron beam is generated utilizing relatively less power than previously known spectrometers.

Another object is to provide such a spectrometer wherein the bounds of the ionization region are accurately controllable.

A further object of the invention is the provision of such a spectrometer which is relatively noise free.

Still another object is to provide a time of flight mass spectrometer wherein the problem of outgassing, which occurs both as a result of the production and of the stopping of the electron beam, is significantly reduced.

Other objects and features of the invention will become apparent to those of ordinary skill in the art as the disclosure is made in the following description of the preferred embodiment of the invention as illustrated in the accompanying sheet of drawings.

BRIEF DESCRIPTION OF THE DRAWING The FIG. shows a block diagram of a preferred embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT The FIGURE shows an evacuable tubular envelope l having sample inlet and outlet conduits l2 and 14, respectively. An electron emitting filament 16 is located at one end of the envelope I0 and is electrically connected to a power supply 18. The filament, in turn, is partially surrounded by a gating electrode 20 which is a quiescently at a sufficient negative potential relative to the filament 16 so as to prevent electrons from passing through the aperture 22 in the gating electrode. The electron multiplier 24, one example of which is disclosed in my copending application Ser. No. 737,490, filed June 17, 1968 is located adjacent to the gating electrode and is coupled via line 26 to high voltage source 28. An open mesh control grid 30, e.g. of lnconel, is coupled through line 32 and beam current monitor 34 to the high voltage source 28 so that the absolute potential on the control grid 30 is approximately 50 volts more positive than the back of the electron multiplier 24 adjacent to the grid. For example, the control grid may be biased to an absolute potential of approximately plus 3000 volts. In addition, an ion focusing electrode 36 is located adjacent to the control grid 30 downstream therefrom and is coupled to a portion of the high voltage source 28 via line 38 so that the potential on the ion focusing electrode is approximately plus 2000 volts.

The envelope 10 has a central drift tube portion 40, which is grounded while the downstream end of the envelope includes an ion detector and electron multiplier 42 which are well known and the use of which is known in prior art spectrometers. This detector and multiplier is positively biased by means of line 44, originating from high voltage source 28, while the output from the detector and multiplier 42 passes into pulse amplifier 46, which is then coupled to the input of a data handling system 48. Such data handling systems for use with time of flight mass spectrometers are per se well known and further description here is unnecessary. A pulsing circuit or pulse former 50 is also coupled to the data handling system 48 and is connected to the gating electrode 20.

In the operation of the time of flight mass spectrometer of this invention, the filament 16 provides a primary source of electrons, with an emission of a few tenths of a microamp typically being used for the greatest sensitivity of the spectrometer. At the same time, the gas which is to be analyzed is introduced through inlet 12 and passes out through outlet 14. It should be understood that the inlet and outlet could be oriented otherwise as represented in the FIGURE. The gating electrode 20 is normally at a sufficiently negative potential relative to the filament 16 so as to prevent electrons from passing through the aperture 22. However, when it is desired to form an ionizing electron beam, the pulse former 50 is caused to generate a positive pulse that is applied to the gating electrode 20 so as to enable a pulse of electrons, approximately 5 nanoseconds wide, to pass through the aperture 22 to the front of the electron multiplier 24. The multiplier then amplifies the current pulse by a factor of approximately 10 and produces a pulse about 10 nanoseconds wide. This is because of the transit time dispersion in the electron multiplier. Thus, a pulse of electrons with a total time spread of less than 15 nanoseconds is produced at the output or back end of the multiplier 24.

Because the potential on the control grid 30 is approximately 50 volts above that at the back of the electron multiplier 24, the electrons emerging therefrom are accelerated toward the control grid 30. These electrons will then oscillate in a potential well present around the grid for several nanoseconds until they are captured by the grid.

The gas to be analyzed is also passing adjacent to the grid 30 and becomes ionized by the electrons as they oscillate about the grid. By controlling the potential on the control grid 30 it can be seen that the distance of travel of the electrons as they oscillate thereabout can be accurately controlled so that the ionization region is likewise controlled. Thus, a very narrow ionization region can be created adjacent to the grid 30 so that ions are formed on the downstream or drift tube side of the grid within a very narrow region. The positive ions are then accelerated by the field present between the control grid 30 and the ground drift tube 40, and a single aperture focusing electrode 36 provides focusing control to assure that all of the ions traverse the drift tube to the ion detector and electron multiplier 42 without striking the walls of the tube.

The structure of this spectrometer is such that the entire ion beam then passes from the ionization region adjacent the control grid 30 to the detector 42 without encountering any additional accelerating electrodes, thus preventing noise which would occur if the ions were to collide with additional electrodes. The only noise that is produced in this spectrometer system is that due to random noise at the ion detector and electron multiplier which is common to all high sensitivity mass spectrometers using an electron multiplier-detector.

As each of the positive ions strikes the detector and multiplier 42 a pulse is produced from the multiplier which passes through pulse amplifier 46 and is then routed to the data handling system 48. The pulse from pulse former 50, which initially triggers the electron beam, also provides a starting signal to the data handling system 48 while the pulse output from amplifier 46, which is related to the reception ofions by detector 42, which is related to the reception ofions by detector 42, provides a signal to the data handling system that is indicative of the time of flight through the drift tube of the various ions that create output pulses from the detector and multiplier 42.

Another problem which is common to all mass spectrometers designed to operate in ultrahigh vacuum is the outgassing which occurs as a result of the production and reception of the electron beam. The spectrometer of this invention signifi cantly reduces this outgassing since the emission from filament 16 is quite low (typically a few tenths ofa microampcre) so that the outgassing from the filament is quite inconsequential. Furthermore, the filament 16 is shielded from the ionization region adjacent to the control grid 30 by the electron multiplier 24 so that it is difficult for any spurious gaseous material outgassed from the filament 16 to interfere with the ionization of the gaseous sample in the ionization region. Even though some outgassing from the filament 16 is to be expected during the time in which the electron pulse is generated at the filament l6 and during the time that the electron beam is traversing the multiplier 24, none of the spurious gas molecules released are able to reach the ionization region during the time of the electron pulse. Thus, to the extent that these spurious gas molecules are able to diffuse out of the ionization region between pulses, they will contribute nothing to the mass spectrum and will not interfere with the accurate operation of the spectrometer. In addition, some outgassing of the control grid 30, which collects the electrons, is to be expected. However, since this grid is continuously bombarded by electrons during operation of the spectrometer it should rapidly reach a steady state condition. If additional degassing of the control grid 30 is required, it can be accomplished by allowing the electron beam to flow continuously for a few seconds by placing a positive DC signal on the gating electrode 20 by means not shown in the FIGURE so as to cause the grid 30 to be bombarded by an average current several thousand times higher than in normal operation.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, the ionizing electron beam can be stabilized against changes in multiplier gain as well as against changes in filament emission by sensing the average current to the control grid 30, e.g. through beam current monitor 34 and by using this signal to servo control the power to filament l6 and to the multiplier 24 from high voltage source 28. In addition,

because the currents required for producing the ionizing electron beam are relatively small, sources other than the heated filament 16 could be used including a light beam or other radiation directed onto a surface to produce secondary electrons. Similarly, it should be understood that various types of data handling systems as are well known in the art could be used. It is therefore to be understood that within the scope of the intended claims the invention may be practiced otherwise than as specifically described.

l claim:

1. Apparatus for analyzing properties of a material sample,

said apparatus comprising:

means for producing a beam of moving electrons along an axial direction,

sampling means for introducing said material sample into said beam ofelectrons,

an open wire mesh disposed substantially transversely to said axial direction,

electrical means operatively connected to said wire mesh for charging said wire mesh to an appropriate electrical potential for producing a potential well about said wire mesh within which said electrons may be accelerated back and forth along said axial direction to bombard said material sample thereby producing an ionization region,

said electrical means including means for controlling the axial dimension of said potential well in said axial direction to thereby substantially control the corresponding axial dimension of said ionization region,

accelerating means for moving said ions away from said ionization region in an ion beam along said axial dimension of said ionization region, and

detector means for detecting said ions and determining the charge to mass ratios of ions produced from said material sample.

2. An apparatus as in claim 1 further comprising:

a tubular vacuum envelope defined by a longitudinal axis passing therethrough and substantially aligned with said electron motion,

said accelerating means including means for biasing said envelope for accelerating said ions out of said ionization region along said longitudinal axis, and

focusing means for accurately controlling the cross-sectional area of said ion beam emanating from said ionization region.

3. An apparatus as in claim 1 wherein said means for producing a beam includes:

a primary source of electrons,

gating means for normally preventing electron passage from said primary source, and

electron multiplier means having an input and an output for receiving and emitting electrons respectively in directions substantially aligned with respect to said direction of electron motion and wherein said multiplier means is disposed between said primary source and said ionization region for increasing the number of electrons available for bombarding said sample material in said ionization region.

4. An apparatus as in claim 1 wherein said detector means comprises:

an ion detector and an electron multiplier means axially aligned with said ion beam for receiving and detecting ions accelerated away from said ionization region. 

1. Apparatus for analyzing properties of a material sample, said apparatus comprising: means for producing a beam of moving electrons along an axial direction, sampling means for introducing said material sample into said beam of electrons, an open wire mesh disposed substantially transversely to said axial direction, electrical means operatively connected to said wire mesh for charging said wire mesh to an appropriate electrical potential for producing a potential well about said wire mesh within which said electrons may be accelerated back and forth along said axial direction to bombard said material sample thereby producing an ionization region, said electrical means including means for controlling the axial dimension of said potential well in said axial direction to thereby substantially control the corresponding axial dimension of said ionization region, accelerating means for moving said ions away from said ionization region in an ion beam along said axial dimension of said ionization region, and detector means for detecting said ions and determining the charge to mass ratios of ions produced from said material sample.
 2. An apparatus as in claim 1 further comprising: a tubular vacuum envelope defined by a longitudinal axis passing therethrough and substantially aligned with said electron motion, said accelerating means including means for biasing said envelope for accelerating said ions out of said ionization region along said longitudinal axis, and focusing means for accurately controlling the cross-sectional area of said ion beam emanating from said ionization region.
 3. An apparatus as in claim 1 wherein said means for producing a beam includes: a primary source of electrons, gating means for normally preventing electron passage from said primary source, and electron multiplier means having an input and an output for receiving and emitting electrons respectively in directions substantially aligned with respect to said direction of electron motion and wherein said multiplier means is disposed between said primary source and said ionization region for increasing the number of electrons available for bomBarding said sample material in said ionization region.
 4. An apparatus as in claim 1 wherein said detector means comprises: an ion detector and an electron multiplier means axially aligned with said ion beam for receiving and detecting ions accelerated away from said ionization region. 